The cell cycle comprises S phase (DNA replication), M phase (mitosis), and two gap phases (G1 and G2 phases) between S and M phases. Checkpoints in the cell cycle ensure accurate progression through cell cycle stages, and include monitoring DNA integrity, DNA replication, cell size, and the surrounding environment (Maller (1991) Curr. Opin. Cell Biol., 3:26). Cell cycle checkpoints that monitor the state of genome include the G1 checkpoint prior to onset of DNA replication and the G2 checkpoint prior to onset of mitosis. The G1 checkpoint allows detection and repair of DNA damage before entering S phase, thereby providing a crucial protective function because, when damaged DNA is replicated, it often gives rise to mutations (Hartwell (1992) Cell 71: 543). The G2 checkpoint allows detection and repair of DNA damage before entering mitosis (M phase), thereby providing a crucial function because mitosis without DNA repair may propagate the damage through DNA-damaged daughter cells, or mitosis may fail entirely. Progression through G1 and G2 checkpoints without repairing extensive DNA damage usually results in cell death.
Inhibition of the cell cycle G2 checkpoint by peptides, peptidomimetics, and “small molecules” has been used to selectively target cancer cells because most cancer cells are defective at one or both of the two major checkpoints of the cell cycle that protect cells from the effects of DNA damage, such that inhibition of the G2 checkpoint allows DNA-damaged cells to re-enter the cell cycle without repairing the DNA damage. (Kawabe T. (2004) “G2 checkpoint abrogators as anti-cancer drugs” Mol Cancer Ther 3: 513-519). Although the molecular mechanism of the cell cycle G2 checkpoint was extensively studied, no single molecular target for the therapeutic G2 checkpoint abrogation was established in earlier studies. A phenotype-based screening protocol has been developed to efficiently identify potential G2 checkpoint inhibitors. (Suganuma M. & Kawabe T., EP Application No. 00964563; Sha et al. (2007) Mol Cancer Ther 6: 147-153), where cell cycle phenotype-based screening of G2 checkpoint abrogating peptides identified CBP501 having a unique mechanism of action at the cell cycle G2 checkpoint. (Sha et al. (2007) Mol Cancer Ther 6: 147-153).